A machine is any device with which work may be
accomplished. In application, machines can be used
for any of the following purposes, or combinations of
these purposes.

1. Machines are used to transform energy, as in
the case of a generator transforming mechanical
energy into electrical energy.

2. Machines are used to transfer energy from one place
to another, as in the examples of the connecting
rods, crankshaft, and reduction gears transferring
energy from an aircraft’s engine to its propeller.

3. Machines are used to multiply force; for example, a
system of pulleys may be used to lift a heavy load.
The pulley system enables the load to be raised by
exerting a force that is smaller than the weight of
the load.

4. Machines can be used to multiply speed. A good
example is the bicycle, by which speed can be
gained by exerting a greater force.

5. Machines can be used to change the direction of
a force. An example of this use is the flag hoist.
A downward force on one side of the rope exerts
an upward force on the other side, raising the flag
toward the top of the pole.

There are only six simple machines. They are the lever,
the pulley, the wheel and axle, the inclined plane, the
screw, and the gear. Physicists, however, recognize
only two basic principles in machines: the lever and
the inclined plane. The pulley (block and tackle), the
wheel and axle, and gears operate on the machine
principle of the lever. The wedge and the screw use
the principle of the inclined plane.

An understanding of the principles of simple machines
provides a necessary foundation for the study of compound
machines, which are combinations of two or
more simple machines.

Mechanical Advantage of Machines

As identified in statements 3 and 4 under simple
machines, a machine can be used to multiply force or
to multiply speed. It cannot, however, multiply force and speed at the same time. In order to gain one, it
must lose the other. To do otherwise would mean the
machine has more power going out than coming in,
and that is not possible.

In reference to machines, mechanical advantage is a
comparison of the output force to the input force, or
the output distance to the input distance. If there is a
mechanical advantage in terms of force, there will be
a fractional disadvantage in terms of distance. The
following formulas can be used to calculate mechanical
advantage.